In an ultraviolet light-emitting element, an n-type semiconductor layer, a light-emitting layer, and a p-type semiconductor layer are stacked in this order on a support substrate. Furthermore, a p-type electrode is formed on the p-type semiconductor layer, and an n-type electrode is formed on the n-type semiconductor layer. When a voltage is applied between the p-type electrode and the n-type electrode of the ultraviolet light-emitting element having such a configuration, holes of the p-type semiconductor layer and electrons of the n-type semiconductor layer recombine in the light-emitting layer, so as to radiate (emit) light corresponding to the band gap of the light-emitting layer. The light emitted by the above-described light-emitting layer is extracted to the outside after passing through the respective layers of the ultraviolet light-emitting element. In case of the ultraviolet light-emitting element having the aforementioned configuration, since the electrodes using Ni, Au, or the like and a p-type contact layer mainly composed of GaN absorb ultraviolet light, almost no light can be extracted from the electrode surface side. Therefore, it is common that the light emitted from the above-described light-emitting layer passes through the n-type semiconductor layer and the support substrate from the light-emitting aver, so that the light is extracted from the support substrate side. In order to efficiently extract light from the support substrate side, it is necessary that the layers of the semiconductor element from the light-emitting layer up to the support substrate and the support substrate have high transmittance with respect to the emission wavelength.
When light passes through the boundary of media having different refractive indices, that is, through layer interface, a surface, or the a certain proportion of light is inevitably reflected. In particular, when light travels from a medium having a large refractive index to a medium having a small refractive index, total reflection of light occurs, so that light incident at a critical angle or greater cannot be extracted to the outside. At the surface of the semiconductor light-emitting element, that is, at the interface between air (or sealing material) and the ultraviolet light-emitting element, since the refractive index difference between the two media becomes large, the critical angle at which total reflection occurs becomes small. As a result, the proportion of light totally reflected at the interface increases, resulting in a problem of deterioration of the light extraction efficiency. For example, if an AlN single crystal with a refractive index of 2.4 is used as the support substrate, the critical angle is calculated to be 24.6° according to Snell's law, and thus, all light with angles of incidence greater than that critical angle is totally reflected. Therefore, it is known that, in a semiconductor light-emitting element in which an AlGaN layer is stacked on an AlN substrate as a light-emitting layer, the extraction efficiency of light that can be extracted from the surface (light extraction surface) side of the AlN substrate is extremely low, i.e., the efficiency is about 4% on the basis of calculation.
Therefore, in order to cope with such a problem, a semiconductor light-emitting element in which an indented or rough surface structure on the nanometer scale is provided on the substrate surface (light extraction surface) has been proposed for The purpose of improving the light extraction efficiency. For example, Patent Literature 1 discloses that an indented structure having an average period of not more than twice the average optical wavelength of light emitted from a light-emitting layer is provided on a light extraction surface. There has been proposed a method of reducing the proportion of the split of the light totally reflected at the light extraction surface (that is, suppressing the reflection of the light on the element surface) by forming such an indented structure. Although the light extraction efficiency is improved by such a method, the light extraction efficiency varies greatly depending on the shape of the indented structure and the emission wavelength Therefore, it cannot be said that sufficiently improved light extraction efficiency is achieved, and further improvement of the light extraction efficiency has been demanded.
From this point of view, various studies have been made on the indented structure to be provided on the substrate surface (light extraction surface). For example, Patent Literature 2 has proposed a semiconductor light-emitting element in which at least one of the light extraction surface and an interface between two layers having different refractive indices in the semiconductor light-emitting element is provided with a periodically indented structure having a period exceeding 0.5 times the wavelength of light emitted from the light-emitting layer and a minutely indented structure located on the surface of the periodically indented structure and having an average diameter of 0.5 times or less the wavelength of light.
In this manner, the total reflection inside the support substrate can be reduced by forming the indented structure on the light extraction surface. However, not all the light can be extracted to the outside because there is light that cannot be extracted to the outside due to the influence of scattering or Fresnel reflection at the interface of the support substrate. When these rays of light reach the light extraction surface again by changing the traveling direction thereof due to interface reflection or the like between the layers of the ultraviolet light-emitting element, some of the rays of light are extracted to the outside in the same manner, and the remaining rays of light are reflected to the layer side of the ultraviolet light-emitting element. As described above, the light which is not extracted to the outside is repeatedly reflected and propagated between the support substrate and the layers of the ultraviolet light-emitting element, but the light intensity is gradually attenuated by absorption in the layers of the ultraviolet light-emitting element and the electrodes, so that the light disappears in time. Therefore, in order to further enhance the light extraction efficiency, it is important to reduce loss of light such as absorption in addition to the formation of the indented structure.
Therefore, Patent Literature 3 proposes a light-emitting element in which an indented portion formed from recessed portions and projecting portions at a pitch larger than the wavelength of the light emitted from the light-emitting layer in the semiconductor layer is provided to the whole or a part of the surface of the ultraviolet light-emitting element from which the light is to be extracted, and a reflective layer using a metal such as aluminum having a high reflectance of the light or a DBR is provided to the surface of the semiconductor layer opposite to the surface thereof from which the light is extracted.